Telephone transmitter



Nov.18, 1947. Q R 2,431,022

TELEPHONE TRANSMITTER Original Filed Dec. 21, 1942 Cf/VE/YTED Patented Nov. 18, 1947 TELEPHONE TRANSMITTER George H. Brodie, La Grange, Ill., assignor to Kellogg Switchboard and Supply Company, Chicago, 111., a corporation of Illinois Original application December 21, 1942, Serial No. 469,767, now Patent No. 2,404,772, dated July 30, 1946. Divided and this application January 31, 1944, Serial No. 520,516

6 Claims. 1

This invention relates to telephone transmitters. Lhe general object is to provide a telephone transmitter of simple and reliable construction which has a substantially uniform output level throughout the frequency range important in telephone transmission.

This application is a division of applicants prior application Serial. No. 469,767, filed December 21, 1942, now Patent No. 2,404,772, granted July 30, 1946,

Generally speaking, the transmitter disclosed herein is a direct improvement on the transmitter disclosed in Patent No. 2,014,427, issuedSepttamber 17, 1935, to George R. Eaton.

A feature of the invention is that an air chamber is enclosed between the transmitter case and the diaphragm, and is divided into front and rear air chambers separated by a partition. By providing suitably sized openings through the par tition, the rear air chamber is caused to cooperate with the front one to permit a more uniform output over the entire frequency range to be obtained.

Other objects and features of the invention will appear more fully as the description progresses.

The accompanying drawings, comprising Figs. 1 to 5, show suflicient views of a transmitter embodying the features of the invention to enable the invention to be understood.

Fig. 1 is a front sectional View taken along the irregular section line of Fig. 2;

Fig. 2 is a bottom view;

Fig. 3 is an exploded view of the parts A to G;

Fig. 4 is a broken-away front sectional view taken along the line 44 of Fig. 2; and

Fig. 5 is a fragmentary front sectional view showing a desirable modification.

Fig. 3 is at a scale of about 1 to 1, while the remaining views are at a scale of about 2 to 1.

The invention is illustrated as applied to what is termed a capsule unit, of the type commonly used in the transmitter opening of a hand telephone.

' Construction In the illustrated construction, the capsule unit is provided with a casing I, illustrated as molded of plastic insulation material. Casing l is provided with a rim 2, used in practice for locating the capsule longitudinally in the transmitter opening of the instrument, and with opposed ribs 3 and 4 which are used to locate the capsule rotationally.

In the disclosed construction, the diaphragm C is preferably stamped from thin sheet metal, and

2 is firmly clamped peripherally by the screwthreaded clamping ring A, notched at 2e and 21 (Figs. 1 and 3) for the insertion of a suitable Wrench.

Diaphragm C is overlaid .by shield disc B, also clamped firmly at the periphery, and having a central opening for the admission of sound waves.

The movable electrode I6 is of generally hemipherical configuration, and. is formed integrally with the diaphragm C.

Fixed electrode G has a generally hemispherical carbon chamber l1 formed therein within which the electrode It lies, the space between the electrodes being occupied by the usual carbon particles. The stem portion of electrode G is held in its desired longitudinal position of adjustment by set screw H, threaded into a side opening through the annular depending boss 5 and the metal insert sleeve 1.

The carbon chamber l! is sealed against the escape of the transmitter material by closure disc F (Figs. 1 and 3). This disc may be formed of any suitable thin sheet material, the material chosen being preferably one having a minimum of the characteristics of spring material. Closely woven silk cloth has been found to be suitable. In the embodiment of the invention shown in Fig. 1, the inner annular portion-of the closure disc F is secured against a shoulder provided about the electrode I 6 by collar D.

The collar D may be made of thermoplastic material, with a slightly undersized inside diameter. The collar D may be heated sufiiciently to render it slightly plastic and then forced upon the base portion of the electrode. When the collar D cools and contracts, it is held firmly in position by the resulting frictional grip around the base of the electrode 16.

The spring, or clamping, disc E, having the illustrated central opening therethrough larger than the outside diameter of collar D, is next assembled over the electrode 16 and collar D, following which the inside edge of closure member F is cemented or otherwise firmly secured to the shoulder formed by the lower end surface of collar D, thus annularly securing the inside portion of the closure member to the movable electrode.

The parts are next assembled as illustrated in Fig. 1, a desired quantity of transmitter material being first placed in chamber ii. The eiiective outside portion of closure disc F lies .on the am nular shoulder formed by the upper end of fixed electrode G, and is firmly clamped against this shoulder by the annular inner portion of clamping disc E. This clamping .disc, being preferably formed of suitable metal, has inherent springiness which becomes effective to grip closure member F securely when the assembly consisting of parts B, C, and E is firmly drawn into position, against the illustrated annular shoulder in casing l, by a tightening of clamping ring A.

The closure disc F is preferably made somewhat larger than the opposed end portion of electrode G on which it is clamped, thereby assuring fully efiective clamping in the event that the outer portion of such closure member is distorted somewhat from a true disc shape because of irregularities in the punching or cutting operation.

External electrical connections to the fixed and movable electrodes, respectively, are afforded by the depending contact clips l2 and I3. Fixed electrode G is held firmly in engagement with insert collar 1 by set screw II, as previously pointed out. Collar I has an annular recess around its upper end into which the ring-like end portion of contact member 8 fits, the two parts being preferably end-staked or otherwise secured together. Contact member 8 has a portion which extends to the rear and then to the left, as seen best in dotted outline in Fig. 2. The rearwardly extending portion of contact member 8 is embedded in the rear extension 6 of depending boss 5, Figs. 2 and 4. The portion of contact member 8 which extends to the left to the mounting position of contact clip [2 is preferably ofiset upwardly as shown in Fig. 4 to bring the lower surface thereof at least within the confines of the lower surfaceof the casing.

It will be understood, of course, that the parts I and 8 are first assembled together, and then incorporated in the casing l during the molding operation. The side extension of member 8 may have a pair of upwardly extending lugs, shown at 9 and (Figs. 2 and 4) which anchor the contact member more securely in the casing. Contact clip l2 may be held firmly in electrical engagement with contact member 8 by a pair of screws, as illustrated, or in any other desired manner.

An electrical circuit path from contact clip [3 to the movable electrode I6 is established by the compression spring 14, which passes through the illustrated vertical opening in the casing I. The lower end of spring l4 contacts the upper surface of clip I3, while the upper end of spring 14, under compression, contacts the lower side of retaining disc E, the disc E and the diaphragm C being held tightly in electrical engagement around their respective peripheries, by ring A.

The modification disclosed in Fig. 5 will now be described. The construction in Fig. 5 may be the same as that in Fig. 1, except that the plastic collar D of Fig. 1 is omitted in Fig. 5, and the shoulder to which the inner portion of closing disc F is secured is formed as a part of the diaphragm C and electrode l6.

An advantage possessed by the arrangement of Fig. 5 is that collar D of Fig. 1 is eliminated, producing a simpler structure and slightly lowering the mass of the diaphragm-electrode assembly. The arrangement of Fig. 1, however, has a somewhat ofisetting advantage in that the same diaphragm C can be used either in the construction of the improved transmitter herein disclosed or as a replacement part in the repair of an existing transmitter of the construction disclosed in the previously noted patent to Eaton.

It is clear that the disclosed construction permits a transmitter to be readily disassembled for 4 inspection or repair, and to be readily reassembled, for the parts B to F are held in assembled position solely by clamp ring A.

Performance characteristics In the construction of a rugged and reliable transmitter, it is considered by many to be imperative that the diaphragm be clamped firmly at its periphery, in the general manner herein disclosed and as disclosed in the previously mentioned Eaton patent. Such construction insures that the diaphragm is maintained permanently in a fixed normal position. The clamping of the diaphragm affords a hermetic seal for the carbon chamber, for any other openings through which air might enter or escape are sealed, as by shellac or the like.

It is an inherent characteristic that a diaphragm clamped around the periphery and carrying an attached electrode is mechanically resonant at a given frequency, causing the transmitter output to peak rather sharply at the point of diaphragm resonance. If uncorrected, this characteristic causes distortion in the transmitted voice currents by accentuating those which are at or near the resonant frequency of the diaphragm.

Two corrective methods have been used. One corrective method is to provide a damping arrangement which is most effective at or near the resonant peak of the diaphragm, while the other is to provide structure affording other resonant peaks spaced more or less uniformly across the frequency range, tending to increase the volume output at points in the frequency range above and below the resonant frequency of the diaphragm.

The acoustic system comprising the rear and front chambers 22 and 23 interconnected by the restricted openings I8 and I9 partakes of both of the above-mentioned corrective methods and it cooperates with other portions of the disclosed transmitter structure to provide a transmitter Whose output remains in the same general volume range throughout the desired frequency range.

The important overall performance characteristics of a typical transmitter constructed in accordance with this disclosure are as given in the following performance table:

In the selected frequency range, from 200 cyles to 3700 cycles, minimum output occurs at about 400 cycles, being about 16.5 db., as given in the above table. The highest output occurs at about 1800 and 2000 cycles, where it is about 28 db., as shown.

The following explanation is given of the action indicated by the above table:

Point 5 Consider first the resonant characteristic of the diaphragm C, with its peak at about 1250 cycles, Point 5 in the table. The output of the transmitter is thereby increased for a considerable band of frequencies extending above and below the resonance peak, as is well known.

Point 3 Consider next the interaction of the chambers 22 and 23 through the restricted openings 18 and 19. These two chambers, acting in combination, appear to reach a resonance peak at Point 3, about 75.0 cycles, as .is indicated in the table. At that point, the transmitter output, controlled by all factors then operative, is about 21 db. When plotted, this point is on. the slope leading up to the diaphragm peak point at 1250 cycles.

The effect of the above-noted 22, .23 peak at 750 cycles, diminishes with rising frequency, as the diaphragm resonance response builds up toward its peak, at 1250 cycles.

Point 4 At Point 6, about 1500 cycles, air pocket 24 (Fig. 1) contained between the protecting shield B and the diaphragm C, reaches its resonant peak. This point lies on the upper declin slope of diaphragm resonance, raising the output to 27 db.

Point 7 At Pointv '7, about 1800 cycles, front chamber 23, then acting largely independently of rear chamber 22, reaches its resonance peak. The output is thereby raised to 23, db., one of the two high points of output.

Point 8 At Point. 8, about 2000 cycles, shield B, acting as a secondary vibratory diaphragm, gives its peak response, the coupling between it and the diaphragm C being directly across the air pocket 24, again raising the output to about 28 db.

Point 9 In practice, the mouthpiece employed with the disclosed transmitter acts as a resonant chamber, with a resonant peak at about 2800 cycles,

at which point the output level is about 27 db. This point is termed the entrance peak.

Point 10 Point 10 in the performance is the upper limit of the selected frequency band. At this point, about 3700 cycles, the output, controlled by all factors there operative, is about 17.5 db.

Point 2 Point 2 in the performance table is the point of minimum output. This point occurs at about 400 cycles, where the output is about 16.5 db. This point is on the lower slope of the 22, 23 peak (Point 3), and is so far down on the slope of the diaphragm peak (Point 5) that the efiect of the latter is very small.

Point 1 Point 1 in the performance table is the lower limit of the selected frequency range. The output there is about 1.7.5 db., or 1 db. higher than the output at Point 2. The difference is believed to result from the compound nature of the rear chamber 22, with an action about as follows.

Rear chamber 22' comprises a main disc-shaped cavity (lying between the lower surface of clamping disc E and the opposed surface of casing I), and the deep-pocket, central space 42, surrounding the main portion of fixed electrode G, between it and the opposed side walls of casing I. It is believed that the 22, 23 peak at Point '3 (about 750 cycles) is influenced mainly by front cavity .23 and the disc-shaped portion of rear cavity .22. It is further believed that both portions 22 and. 42 of the rear cavity cooperate with the front cavity 23, through restricted openings 18 and I9 to provide a resonance peak somewhere below '200 cycles. Point 1 of the performance table lies on the upper slope of the lastnamed peak raising the output at that point to 17 db.

By changing'the volume and depth of the deep pocket 42, the general output level below Point 3 in the performance table can be raised or lowered, within limits, with negligible eifect on the output from Points 3 to 10.

In the illustrated construction, best results have been obtained when opening l8 and I9 are each about .019 inch in diameter. With substantially smaller openings, the effect f the rear chamber 22 begins to be lost, with resulting distortion in the output performance. When openings I8 and I9 are made considerably larger than the aboveindicated size, the comparatively free communication then resulting between the two chambers 22 and 23 causes the action to approach that of only a single large chamber behind the diaphragm, resulting again in a distorted output.

I claim:

1. In an acoustic device, a casing, a diaphragm mounted on said casin and having a natural period of vibration, said casing containing a closed chamber adjacent said diaphragm, and means separating said chamber into a first portion adjacent the diaphragm and a second portion separated from the diaphragm, there being a restricted passageway interconnecting said portions, the first portion of said chamber attaining resonance at a frequency substantially above the natural period of the diaphragm to thereby reinforce the diaphragm movement at such frequency, the two portions, interacting through said restricted passageway, attaining anti-resonance at a frequency near the natural period of the diaphragm to thereby damp the natural period movement of the diaphragm; the two portions, interacting through said restricted passageway, attaining resonance at each of two frequencies substantially below the natural period of the diaphragm to thereby reinforce the diaphragm movement at such frequencies, the second portion of said chamber including a shallow part and a deep part.

2. In a telephone transmitter, a casing, a diaphragm mounted on the casing, said casing containing a closed chamber lying on one side of the diaphragm, an annulus extending across the chamber and dividing it into a front portion adjacent to the diaphragm and a rear portion remote from the diaphragm, a rear electrode member lying bodily within the confines of the rear portion of the chamber and fixed, at its rear portion, with the casing, there being an annular passageway between the front and rear portions of the chamber defined by the front portion of the rear electrode and the inner portion of the annulus, material of the class including silk cloth disposed across said passageway, and a front electrode fixed with the diaphragm and coacting microphonically with the rear electrode.

3. In a telephone transmitter, a casing, a diaphragm mounted on the casing, said casingcontaining a closed chamber lying on one side of the diaphragm, an annulus extending across the chamber and dividing it into a front portion adjacent to the diaphragm and a rear portion remote from the diaphragm, a rear electrode member lying bodily within the confine of the rear portion of the chamber and fixed, at its rear portion, with the casing, there being an annular passageway between the front and rear portions of the chamber defined by the front portion of the rear electrode and the inner portion of the annulus, material of the class including silk cloth disposed across said passageway, and a front electrode fixed with the diaphragm and ooacting microphonically with the rear electrode through the annulus.

4. In a telephone transmitter, a casing, a diaphragm mounted thereon, said casing containing a sealed chamber lying on one side of the diaphragm, an annulus extending across the chamber, a rear electrode member fixed with the casing, and a front electrode fixed with the diaphragm and coacting microphonically with the rear electrode, said annulus and rear electrode member being so dimensioned and located as jointly to divide the chamber into a front portion adjacent to the diaphragm and a rear portion remote from the diaphragm, there being an acoustic passageway between the front and rear portions of the chamber.

5. In a telephone transmitter, a casing, a diaphragm mounted thereon, said casing containing a sealed chamber lying on one side of the diaphragm, an annulus extending across the chamher, a rear electrode member fixed with the casing, and a front electrode member fixed with the diaphragm and coacting microphonically with the rear electrode, said annulus and rear electrode member being so dimensioned and located as jointly to divide the chamber into a front portion adjacent to the diaphragm and a rear portion remote from the diaphragm, there being an acoustic passageway between the front and rear portions of the chamber comprising a perforation of an intermediate portion of the annulus.

6. In a telephone transmitter, a casing containing a chamber, a diaphragm mounted on the casing and serving to close one side of the chamber, a front electrode fixed with the diaphragm and lying within the chamber, a rear electrode fixed with the casing, an annulu extending across the chamber between the diaphragm and the rear electrode, the annulus cooperating with the casing and the rear electrode to divide the principal part of the chamber into a front portion adjacent to the diaphragm and a rear portion remote from the diaphragm, the front electrode extending through the opening in the annulus into cooperative relationship with the rear electrode, the space between the said electrodes comprising a third portion of the chamber.

GEORGE H. BRODIE.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number 

